1. Field of the Invention
This invention relates to a photoelectric conversion system having a photoelectric conversion device and a detection circuit for detecting an output signal of the photoelectric conversion device.
2. Description of the Related Art
As a photoelectric conversion device, a MOSFET photocell and photodiode using monocrystalline silicon, amorphous silicon, polycrystallinesilicon or the like are known. A CCD is well known in the art as a system for transferring charges generated in the silicon bulk by application of light by scanning an electric field. Because of the charge transfer inside the bulk, in this system, a so-called semiconductor thin film deposition technique for forming a photoelectric conversion device on the insulating substrate cannot be used and this system is not suitable for a photo device of large area. Further, when illumination light is intense, surplus charges are leaked from a pixel portion to the adjacent pixel portion to cause a smear phenomenon or blooming phenomenon that a pixel of black level is observed as a whitish pixel.
An area type photo-array can be formed by arranging photoelectric conversion devices constructed by photodiodes or MOS photocells on the insulating substrate in a matrix form. In this case, charges stored on the respective photoelectric conversion devices by application of illumination light are sequentially derived out at preset timings by effecting the scanning operations in the horizontal and vertical directions. The derivedout charge is amplified by a video amplifier and subjected to the A/D conversion process. Then, gradation display is effected according to the result of the A/D conversion process. Since this type can be made in a large area form and each pixel portion can be made in an island form which is independent from the adjacent pixel portion, the blooming phenomenon and smear phenomenon can be prevented. In order to read out charges generated in the photodiode or MOS photocell by application of illumination light, it is necessary to amplify the charge and then subject the same to the A/D conversion.
A novel photosensor with TFT structure is proposed by the inventor of this application. This invention is disclosed in Japanese Patent Application KOKAI publication No. 3-82171, and the outline thereof is explained with reference to FIGS. 14 and 15.
FIG. 14 is an enlarged cross sectional view of a TFT photosensor 1. The photosensor 1 has a structure having a bottom gate electrode 3, bottom gate insulating film 4, photoelectric conversion semiconductor layer 5 formed of amorphous silicon, source electrode 6, drain electrode 7, top gate insulating film 8 and top gate electrode 9 laminated in this order on a transparent glass substrate 2. The top gate electrode 9 and top gate insulating film 8 are transparent. The source electrode 6 and drain electrode 7 are separated from each other so that light can be illuminated on that portion of the semiconductor layer 5 which lies between the edge portions of the electrodes. Illumination light L is illuminated from the top gate electrode 9 side of the photosensor 1. The operation of the photosensor 1 is explained with reference to FIG. 15.
FIG. 15 shows characteristic curves showing the relation between a drain current I.sub.D and a top gate voltage V.sub.TG applied to the top gate electrode 9 using the presence and absence of the illumination light L as parameters in a condition that a bottom gate voltage V.sub.BG =+20 V is applied to the bottom gate electrode 3 and a drain voltage V.sub.d =+10 V is applied between the source electrode 6 and the drain electrode 7. In FIG. 15, the characteristic curve C.sub.O indicates a case of no light illumination and the characteristic curve C.sub.L indicates a case of light illumination.
In the case of no light illumination, n-channels are formed in both of the upper and lower surface layers of the semiconductor layer 5 when the top gate voltage V.sub.TG =+40 V. As a result, a drain current I.sub.D of several tens micro A (Ampere) can be obtained. The drain current I.sub.D becomes smaller as the top gate voltage V.sub.TG becomes lower, and it becomes smaller than 10.sup.-14 A when the top gate voltage V.sub.TG becomes approximately equal to -20 V. This is considered to be because the n-channel formed in the lower surface of the semiconductor layer 5 by application of the bottom gate voltage V.sub.BG =+20 V is cancelled by application of the top gate voltage V.sub.TG =-20 V. In the case of light illumination, a drain current I.sub.D which is as large as several tens micro A as in the case of no light illumination flows at the time of application of the top gate voltage V.sub.TG =+40 V as might be expected. Unlike the case of no light illumination, the drain current I.sub.D will not be significantly reduced when the top gate voltage V.sub.TG is lowered and a current of approx. 1 micro A will flow even when the top gate voltage V.sub.TG is lowered to -40 V. Therefore, in the photosensor 1, an excellent characteristic in which the ratio of a current (light current) in the case of light illumination to a current (dark current) in the case of no light illumination is set to a number in seven figures can be obtained.
However, even in the above photosensor, only the signal-to-noise ratio can be increased, and like the conventional photodiode and MOS photocell, in order to read out stored charges, it is required to amplify the charge by use of a video amplifier and subject the same to the A/D conversion process. As a result, the sense amplifier is made complicated in construction and the sense amplifier cannot be formed without using a monocrystalline bulk at the present stage and becomes expensive and large in size.
This invention is made in the above-described situations and an object of this invention is to provide a novel photoelectric conversion system which can be designed to detect a current generated in the photoelectric conversion device by application of illumination light by use of a circuit of simple construction.